Effect of basic site substituents on concerted proton-electron transfer in hydrogen-bonded pyridyl-phenols

摘要

Separated concerted proton-electron transfer (sCPET) reactions of two series of phenols with pendent substituted pyridyl moieties are described. The pyridine is either attached directly to the phenol (HOAr-pyX) or connected through a methylene linker (HOArCH_2pyX) (X = 4-NO_2, 5-CF_3, 4-CH_3, and 4-NMe_2). Electron-donating and -withdrawing substituents have a substantial effect on the chemical environment of the transferring proton, as indicated by IR and 1H NMR spectra, X-ray structures, and computational studies. One-electron oxidation of the phenols occurs concomitantly with proton transfer from the phenolic oxygen to the pyridyl nitrogen. The oxidation potentials vary linearly with the pK _a of the free pyridine (pyX), with slopes slightly below the Nerstian value of 59 mV/pK_a. For the HOArCH_2pyX series, the rate constants ksCPET for oxidation by NAr_3 ~(?+) or [Fe(diimine)_3]~(3+) vary primarily with the thermodynamic driving force (ΔG° sCPET), whether ΔG° is changed by varying the potential of the oxidant or the substituent on the pyridine, indicating a constant intrinsic barrier λ. In contrast, the substituents in the HOAr-pyX series affect λ as well as ΔG°sCPET, and compounds with electron-withdrawing substituents have significantly lower reactivity. The relationship between the structural and spectroscopic properties of the phenols and their CPET reactivity is discussed.